Transistor devices such as metal-oxide-semiconductor field-effect transistors (MOSFET), bipolar junction transistors (BJT) or insulated-gate bipolar transistors (IGBT) are used in a wide variety of applications. In particular, IGBT transistors provide fast switching, high current, and high voltage capabilities making such transistors ideally suited for switch-mode power conversion applications.
Transistors are typically operated using a driver connected to the transistor. For instance, most IGBT transistors have three terminals referred to as gate, collector, and emitter. The ON or OFF state of the IGBT transistor can be controlled by modulating the gate-to-emitter voltage VGE via a gate driver circuit. When the voltage across VGE exceeds a threshold voltage Vth, the IGBT may be placed in the ON state (close), allowing a current, also referred to as switch-current, to flow through the collector and emitter terminals. Likewise, when the voltage across VGE is below Vth, the IGBT may be placed in the OFF state (open), and the transistor does not conduct.
A transistor may be operated using different turn-on approaches referred to as hard and soft. When driving the transistor with a hard turn-on, the voltage VCE is positive when the gate voltage VG is greater than Vth. When driving the transistor with a soft turn-on, the voltage VCE is negative or null when the gate voltage VG is greater than Vth.
Driving the transistor with a hard turn-on may lead to large collector current IC spikes. When VCE is high enough for example greater than 5V, the saturation current of the transistor is a function of VGE. Small variations in VGE can lead to large variations in IC. For instance, while IC is about 25 A with VGE=9V, the current IC can exceed one hundred amps with VGE>12V. Such large IC spikes can degrade the reliability of the transistor itself or of other components present in the circuit, especially if IC exceeds the safe pulse collector current rating. In addition, large and fast current spikes can increase electromagnetic interferences.